Prometheus metrics plugin is loaded but causes NPE

We think we've found a regression with the Broker 7.7 release after running the AMQ Online test suite on the final broker image. It appears that prometheus is unable to scrape broker metrics. The broker logs show the following error:

java.lang.NullPointerException: null
        at 
org.apache.activemq.artemis.core.server.metrics.plugins.ArtemisPrometheusMetricsPluginServlet.doGet(ArtemisPrometheusMetricsPluginServlet.java:49)

This NPE is fixed by https://github.com/rh-messaging/artemis-prometheus-metrics-plugin/pull/2  - however it does not fix the underlying problem of metrics plugin not working.

I've been able to reproduce this with the broker 7.7 distribution outside of OpenShift and following the documention at https://access.redhat.com/documentation/en-us/red_hat_amq/7.7/html/managing_amq_broker/prometheus-plugin-managing for configuring the plugin.

I've attached broker.xml and the broker.log. I've noticed that the broker seems to be able to load the plugin at startup:

 2020-06-30 11:58:56,528 INFO  [org.apache.activemq.artemis.core.server] AMQ221082: Initializing metrics plugin org.apache.activemq.artemis.core.server.metrics.plugins.ArtemisPrometheusMetric
sPlugin with properties: {} 

The exception is printed when I'm trying to access localhost:8161/metrics.

broker.xml

^broker.log